Influence of the synthetic pyrethroid insecticide alpha-cypermethrin on the structure of the syrphid coenosis (Syrphidae, Diptera) in a potato ecosystem.

The influence of the synthetic pyrethroid insecticide alpha-cypermethrin on the structure of syrphid coenosis was assessed in a field experiment. A significant influence on the number of species was noticed and a lesser influence upon the species composition of the syrphid coenosis. The effect of the chemical mixture on the population density of the distinct species differed in intensity but only on the first day could the effect of the insecticide be shown with regard to number complex before and after treatment. The total average density, defined as the result of all densities of the distinct species of syrphid flies was most highly affected on the first day after the treatment. At that time the estimated density was half than the level before the application of the insecticide. The effect of the insecticide treatment was manifested upon both the composition of complexes of dominant species and the general dominant structure of the syrphid coenosis. The parameters examined for species structure: Simpson's index of concentration of domination (D), Shannon-Weaver's index of total diversity (H) and Margalef s index of species abundance (d) showed that the insecticide treatment exerts an unfavourable but not strong influence upon the syrphid coenosis. [ABSTRACT FROM AUTHOR]